1. Field of the Invention
The present invention relates to a two-photon absorption dye-containing material which has a great off-resonant two-photon absorption cross section and contains an off-resonant two-photon absorption dye capable of decoloring itself, and to a three-dimensional refractive index or absorption index modulation material and a three-dimensional optical recording material which each utilize the aforesaid decoloring material.
2. Background Art
In general, nonlinear optical effects are nonlinear optical responses that are proportional to an applied photoelectric field raised to the second power, the third power or the power of higher order. Known second-order nonlinear optical effects proportional to the square of an applied photoelectric field include second-harmonic generation (SHG), photo-rectification, photorefractive effect, Pockels effect, parametric amplification, parametric oscillation, sum-frequency photomixing and difference-frequency photomixing. And known third-order nonlinear optical effects proportional to the cube of an applied photoelectric field include third-harmonic generation, optical Kerr effect, self-induced refractive index change and two-photon absorption.
A great many inorganic materials have so far been found functioning as nonlinear optical materials showing those nonlinear optical effects. However, inorganic substances are difficult to prepare the so-called molecular designs for optimization of the intended nonlinear optical characteristics and various physical properties required for device fabrication, so that it is very difficult to put them to practical use. On the other hand, organic compounds permit molecular designs for not only optimization of the intended nonlinear optical characteristics but also control of other various physical properties, so they have high practicability and attention is being given to them as promising nonlinear optical materials.
Of the nonlinear optical characteristics of organic compounds, the third-order nonlinear optical effects have attracted attention in recent years. Among all those effects, particular attention is focused on off-resonant two-photon absorption. The term “two-photon absorption” refers to a phenomenon that a compound is excited by simultaneous absorption of two photons, and the term “off-resonant two-photon absorption” indicates a case where two-photon absorption takes place in an energy region in which a compound has no (linear) absorption band. Incidentally, the term “two-photon absorption” as used in the following description signifies the off-resonant two-photon absorption unless otherwise specified.
Additionally, the probability of off-resonant two-photon absorption is proportional to the square of a photoelectric field applied (square characteristic of two-photon absorption). When a two-dimensional plane is irradiated with laser, therefore, absorption of two photons takes place only at the position of high electric field strength in the center of a laser spot, and it does not take place at all on the periphery area where electric field strength is weak. In a three-dimensional space, on the other hand, two-photon absorption takes place only in the region of great electric field strength in a focus of a lens by which laser beams are gathered, and it does not take place at all in the out-of-focus region because the electric field strength is weak in such a region. In contrast to linear absorption originating in excitation taking place at every position in proportion to the strength of a photoelectric field applied, off-resonant two-photon absorption is characterized in that excitation takes place at only one point in the interior of a space because this absorption has the square characteristic; as a result, remarkable enhancement of spatial resolution can be achieved.
For induction of off-resonant two-photon absorption, short-pulse laser of wavelengths in the near-infrared region where no absorption by a compound is present, which are longer than those in the region where (linear) absorption bands of the compound are present, is used in many general cases. As near-infrared rays in the so-called transparent region are used, exciting light can reach to the interior of a sample without undergoing absorption and scattering and excite one point inside the sample with an extremely high spatial resolution owing to the square characteristic of off-resonant two-photon absorption.
On the other hand, optical information recording media (optical disks) on which information is recordable only once by means of laser light have so far been known, and write-once CDs (the so-called CD-Rs) and write-once DVDs (the so-called DVD-Rs) have been brought to the commercial stage.
For instance, a typical structure of DVD-R is made up of a transparent disk-form substrate in which a pregroove narrowed to no more than one-half (0,74 to 0.8 μm) that of CD-R is cut for tracking laser light applied, a dye-containing recording layer, a light reflection layer generally provided on the recording layer, and further a protective layer as required.
Information is recorded on a DVD-R by irradiating the DVD-R with visible laser light (generally ranging in wavelength from 630 nm to 680 nm). Upon the irradiation, the irradiated area of the recording layer absorbs the light and causes a local rise in temperature to undergo a physical or chemical change (e.g., formation of pits), and by extension to a change in optical characteristics; as a result, a record of the change is kept therein. On the other hand, reading (reproducing or playback) of the recorded information is carried out also by irradiation with laser light of the same wavelength as the laser light used for recording has, and the information is played back by detecting reflectivity differences between the optical characteristic-changed regions of the recording layer (recorded regions) and the unchanged regions of the recording layer (unrecorded regions). As these reflectivity differences are based on the so-called refractive index modulation, greater differences in refractive index between recorded regions and unrecorded regions result in the greater reflectivity ratios of light, namely the greater S/N ratios favorable for playback.
Resent years have seen proliferations of communications network, such as the Internet, and Hi-Vision (HDTV, or High Definition Television). In addition, HDTV broadcasts are imminent, and there is the growing need for consumer-oriented large-capacity recording mediums on which at least 50 gigabytes, preferably at least 100 gigabytes, of image information can be simply recorded at low cost.
Further, for business uses, such as a computer backup use and a broadcast backup use, optical recording medium on which bulk information of the order of 1 terabytes or above can be recorded at high speed and low cost are in increasing demand.
However, two-dimensional optical recording mediums currently in use, such as DVD-Rs, have capacities of the order of at most 25 gigabytes from their physical principle even when the wavelengths of light for record and playback are shortened. Accordingly, those recording mediums have a situation in which it cannot be said that they promise to deliver recording capacities large enough to meet future requirements.
Under these circumstances, attention has focused suddenly on three-dimensional optical recording mediums as ultimate high-density, large-capacity recording mediums. By stacking tens or hundreds layers of record in the direction of the third dimension (layer thickness), the three-dimensional optical recording medium are intended for achievement of recording with ultra-high density and capacity increased by a factor of tens or hundreds, compared with those of two-dimensional optical recording mediums currently in use. For making the three-dimensional recording medium available, the ability to access arbitrary points in the direction of the third order (layer thickness) and write thereon is essential. As a means of access and write, the method of using a two-photon absorption material or the method of using holography (interference) can be adopted.
The three-dimensional optical recording medium using two-photon absorption materials enable the so-called bit recording multiplied by a factor of tens or hundreds on the basis of the physical principle mentioned above, and pave the road to higher-density recording. Therefore, it can be said that they are just the ultimate high-density, large-capacity optical recording medium.
For achieving three-dimensional optical recording by use of two-photon absorption materials, the methods in which fluorescent materials are used for record and playback and their fluorescence is utilized for reading (JP-T-2001-524245 to Levich, Eugene, Boris, et al. (the term “JP-T” as used herein means a published Japanese translation of a PCT patent application), and JP-T-2000-512061 to Pavel, Eugen, et al.) and the methods in which absorption by photochromic compounds or fluorescence from them is utilized for reading (JP-T-2001-522119 to Koroteev, Nicolai, I, et al., and JP-T-2001-508221 to Arsenov, Vladimir, et al.) have been proposed. Therein, however, no two-photon absorption materials are presented specifically, but the compound examples abstractly presented are two-photon absorption compounds having extremely low efficiencies of two-photon absorption. In addition, those methods have problems with nondestructive read, long-term storage of recordings and SIN ratios during playback, and so it cannot be said that they are practical for optical recording.
From the viewpoint of nondestructive read and long-term storage of recordings in particular, utilization of reflectivity (refractive index) changes caused in irreversible materials is desirable for playback. However, there have been no cases of specifically disclosing two-photon absorption materials having such capabilities.
The recording devices capable of three-dimensional recording by refractive index modulation, and the playback devices and the reading methods applied thereto, are disclosed in JP-A-6-28672 to Satoshi Kawada and Yoshimasa Kawada and JP-A-6-118306 to Satoshi Kawada, Yoshimasa Kawada, et al., but these documents have no description of a method of using a two-photon absorption dye-containing material.